Mix the yogurt sauce. Before handling raw beef, stir the yogurt, crushed garlic, and sauce salt together. Cover and refrigerate until serving.
Prepare the filling. Mix the beef, very finely diced onion, measured cold filling water, filling salt, and pepper just until evenly distributed. Do not pre-cook the beef. Cover and refrigerate; take out only the portion needed while shaping. Wash hands, tools, and work surfaces after raw-meat handling.
Knead the dough. Mix the flour and dough salt, then add the lukewarm dough water and dough oil. Knead for about 8–10 minutes until smooth, elastic, and manageable, with no dry flour pockets. Keep the measured 56% hydration as the starting point; this is not a wet flatbread dough.
Prepare the brushing mixture. Combine the melted, just-warm butter with the separate layering oil. Reserve a little of this mixture for lightly coating the dough balls and pans; keep a substantial portion for brushing the formed pastries. Do not add an unmeasured extra pool of fat.
Divide and rest. Divide into equal dough balls using the scale-specific batch guide in Notes. Lightly coat, cover completely, and rest for about 30 minutes. The dough relaxes rather than rising; it contains no yeast.
Build the stacks. Roll each ball into a round about 7–8 inches (18–20 cm) wide on a minimally floured surface. Make stacks of three rounds each, brushing the lower two rounds with a thin film of the butter-oil mixture. Keep enough mixture for the pans and final brushing. Cover the stacks and rest another 10 minutes. Preheat a conventional oven to 410°F (210°C) during this stage.
Roll and cut. Keep the other stacks covered while rolling one stack to a thin rectangle about 16 × 20 inches (40 × 50 cm). Cut about 2-inch (5 cm) squares. Keep cut dough covered so the borders stay flexible. Repeat with the remaining stacks; retain usable edge pieces rather than assuming a perfectly measured yield.
Portion the filling. Count the usable squares and divide the chilled filling proportionally among them. For the stated small squares, about 4.6–5.2 g filling per piece is a practical calculated starting range, not a heaped teaspoon. Test-close a few pieces first, leaving clean borders; use the actual count and the Notes calculation to avoid a large leftover mound of filling.
Close and arrange. Bring two opposite corners together above the filling, then bring the other corners to the center. Pinch the side openings tightly closed. Set seam-side down in lightly coated metal pans, compactly but without crushing or stretching neighboring parcels. Use additional pans if needed. Keep formed raw pastries and unused filling refrigerated while other batches are shaped.
Brush and bake. Brush the tops and gaps with the remaining butter-oil mixture. Bake in the fully preheated oven for about 30–35 minutes, until golden and the dough is cooked through. With two pans, use separated upper- and lower-middle racks, swap rack positions and rotate front to back after about 20 minutes. Check representative pastries from the middle of each pan with a thin thermometer probe: the beef filling must reach at least 160°F (71°C). Continue baking if necessary; browning alone is not a safety check.
Rest briefly. Rest for about 10 minutes after baking. Loosely cover with a clean towel for softer tops, or leave uncovered for a crisper surface. Do not cover with a towel inside the oven.
Serve with cold yogurt. Divide the cooked pastries and sauce into the servings shown. Serve the hot mantije with the refrigerated garlic yogurt alongside or spooned over just before eating. Do not soak the whole tray in sauce far in advance.